The present invention relates to an air separation method and system for producing oxygen to support combustion of a fuel. More particularly, the present invention relates to such a method and system in which the combustion produces heat for a heat consuming device. Even more particularly, the present invention relates to such a method and system in which the air is heated within the heat consuming device and then separated within a ceramic membrane separation system to produce the oxygen.
There are growing concerns about environmental issues arising from the emission of pollutants produced by fossil fuel fired combustion systems. Such combustion systems represent one of the largest sources of carbon dioxide in air pollution emissions. It is known that an effective way to reduce such emissions and to increase the efficiency of combustion is to use oxygen or oxygen-enriched air within the combustion process. The use of oxygen or oxygen-enriched air reduces stack heat losses, which increases the system efficiency, while at the same time reducing NOx emissions. Additionally, the concentration of carbon dioxide in the flue gas is higher since there is little or no nitrogen to act as a diluent. Such flue gas can be more readily used to produce a carbon dioxide rich stream for reuse or sequestration than flue gas having a high nitrogen content.
The use of oxygen to support combustion has found application in processes that require high temperatures, for instance, glass furnaces. In such applications, the fuel savings and other benefits achieved outweigh the cost of the oxygen. When air is used to support the combustion of the fuel for such high temperature applications, a significant part of the heating value of the fuel is expended in the heating of nitrogen contained within the air. This heat is then wasted when the resultant flue gas is exhausted at high temperatures. In low temperature exhaust systems, such as boilers, the resultant heat loss is much lower since more heat is recovered from the flue gas before it is exhausted to the atmosphere. Thus, in this case the use of oxygen is economically unattractive because the cost of the oxygen is greater than any available savings to be realized with reduced fuel consumption. In fact, when the energy required to conventionally produce the oxygen by known cryogenic and adsorptive processes is considered, the overall thermal efficiency decreases.
A major alternative to cryogenic or adsorptive production of oxygen is on-site production of oxygen through oxygen-selective, ion conducting ceramic membrane systems. In such systems, the membrane itself is impermeable to oxygen. The oxygen is compressed and ionized at one surface of the ceramic membrane. The oxygen ions are conducted through the membrane and recombined to form oxygen molecules. In the recombination, electrons are given up by the oxygen ions and either travel directly through the membrane or through a conductive pathway to ionize the oxygen at the opposite surface of the membrane. Such ceramic membranes conduct ions at high temperatures that can reach over 1000xc2x0 C. Thus, in the prior art, auxiliary combustion is used to provide the high requisite operational temperatures of the ceramic membrane.
For instance, U.S. Pat. No. 5,888,272 discloses a process in which oxygen is separated from a compressed feed-gas stream in a transport module-combustor in which separated oxygen is used to support combustion of a fuel to produce the high operational temperatures for the membrane. In one embodiment, a membrane permeate stream is used in a downstream heat consuming process that produces an exhaust which is used to purge the permeate side of the membrane. The permeate stream can also be used to support combustion in an external combustor that is situated upstream of the heat consuming process. Part of the combustor exhaust can be used to supply additional purge gases. A portion of the relatively cool exhaust of the heat consuming process together with the heated retentate is used to heat the incoming air in an external heat exchanger.
U.S. Pat. No. 5,855,648 discloses a process to produce oxygen-enriched feed gas stream to be fed into a blast furnace. In accordance with this patent, air is compressed and heated. Part of the air, after having been heated in an external heat exchanger, is introduced into a ceramic membrane system to produce a permeate stream. The permeate stream is in turn introduced into the incoming heated air stream and used to make oxygen-enriched air for introduction into the furnace. A fuel can be added to the air to be separated in the ceramic me ran to support combustion within the membrane itself. Additionally, part of the compression energy can be recovered with an expander.
Although both patents contemplate an integration of a ceramic membrane system with a heat consuming device, neither contemplate a complete thermal integration of completely independent operating systems. For instance, in U.S. Pat. No. 5,888,272 even where oxy-fuel combustion is contemplated, the combustion and oxygen production are integral components, thus making it very difficult to operate such a heat consuming device without the oxygen production system. Further, the heating of air within an external heat exchanger through heat exchange with exhaust gases of the heat consuming process is inefficient in that there are invariably heat losses to the environment with such an arrangement. While U.S. Pat. No. 5,855,648 contemplates oxygen-enriched combustion within the heat consuming process itself, namely the blast furnace, the hot exhaust gases from such process are expelled without any provision for recovery of their heating value.
In both of the foregoing patents, the ceramic membrane system is utilized in processes in which combustion gases come in contact with the membrane. As such, both patents have limited application to the use of fuels having a high inorganic content, such as coal and heavy oil. Since cleaner fuel such as natural gas is generally more expensive than fuels having a high inorganic content, it is desirable to have a process and system that can be integrated with fuels with a high inorganic content.
As will be discussed, the present invention provides a method and system for oxygen or oxygen enhanced combustion within a heat consuming device that efficiently utilizes a ceramic membrane system to supply the oxygen. Further, a method and system in accordance with the present invention has applicability to low temperature exhaust systems such as a boiler or furnace and is readily capable of using fuels with a high inorganic content. Still further, such a system is designed such that the heat consuming device can be operated without the ceramic membrane if required.
The present invention provides a method of separating oxygen from air for producing oxygen to support combustion of a fuel, thereby to produce heat in a heat consuming device. In accordance with the method, a feed air stream is compressed to produce a compressed air stream. The compressed air stream is heated to an operational temperature of a ceramic membrane system employing at least one oxygen selective, ion conducting membrane. The compressed air stream is heated through indirect heat exchange and at least in part within the heat consuming device. After having been heated, the compressed air stream is introduced into the membrane system to produce an oxygen permeate and an oxygen depleted retentate. The fuel is burned in the presence of an oxidant made up at least in part from the oxygen permeate produced within the membrane system.
Advantageously, a retentate stream composed of the retentate can be expanded with the production of work and the work of expansion can be applied to compress the feed air stream. The expansion of the retentate stream produces an expanded retentate stream which can be used to pre-heat the compressed air stream through indirect heat exchange. Additionally, the feed air stream can be compressed to a pressure sufficient to drive the separation of oxygen from the air within the ceramic membrane system without for instance, the use of a purge stream.
In the heat consuming device a burnerproduces heated flue gases from the combustion of the fuel. The compressed air stream can be heated within the heat consuming device through indirect heat exchange with this heated flue gas. Alternatively, the heat consuming device is the type that is provided with a radiant heat exchange zone and the compressed air stream is primarily heated within the heat consuming device by radiant heat within the radiant heat exchange zone.
In another aspect, the present invention provides an air separation system for producing oxygen to support combustion of a fuel and thereby to produce heat in a heat consuming device. In accordance with this aspect of the present invention, a compressor is provided to compress a feed air stream and thereby to produce a compressed air stream. A ceramic membrane system is in communication with the compressor and employs at least one oxygen selective, ion conducting membrane to separate oxygen from the compressed feed air stream. A heat exchanger, located within the heat consuming device, is interposed between the compressor and the membrane system to heat the compressed air stream to an operational temperature of the membrane system. A means is provided for burning the fuel within the heat consuming device in the presence of an oxidant made up at least in part from the oxygen permeate produced within the membrane system.
Advantageously, the air separation system may employ an expander connected to the membrane system for expanding a retentate stream composed of the retentate with the production of work. A means can be provided for applying the work of expansion to power the compressor. A pre-heater can be interposed between the heat exchanger and the compressor and connected to the expander to pre-heat the compressed air stream through indirect heat exchange with an expanded retentate stream produced by the expander.
The fuel burning means can produce heated flue gases from the combustion of the fuel and the heat exchanger can be positioned within the heat consuming device such that the compressed air stream is heated through indirect heat exchange with the heated flue gases. Alternatively, the heat consuming device can be of the type provided with a radiant heat exchange zone and the heat exchanger can be located within the radiant heat exchange zone such that the compressed air stream is primarily heated by radiant heat within the radiant heat exchange zone. As may be appreciated, such a heat consuming device can be a boiler and the heat exchanger can comprise heat exchange tubes interspersed with steam tubes.
As is apparent from the above description of the present invention, an integration is contemplated in which the ceramic membrane system is thermally integrated with a heat consuming device that employs oxygen-enhanced combustion through oxygen produced within the ceramic membrane system. It should be noted that the placement of a heat exchanger within the heat consuming device would at first appear to be counterproductive or, at best, provide no additional benefits as compared to a conventional air-fired device.
In both the present invention and the prior-art air-fired case, air is heated as part of operating the heat consuming device. In conventional air-fired devices the air heating, in particular the inert components of the air, such as nitrogen, are heated within the combustion space. In the present invention the air heating is done indirectly through the use of heat exhangers. The configuration of the present invention would therefore, at first glance, appear to be identical to the conventional air fired case from the standpoint of thermal efficiency. Upon further examination, thermal efficiency of the present invention might in fact be expected to be less than that of the air fired case due to inevitable, environmental heat losses that are occasioned by the use of external heat exchange and the piping of heated permeate streams to the heat consuming device. However, the present invention actually provides a significant increase in efficiencies over prior art air fired cases. Typical air-fired boilers are anywhere from about 85% to about 90% efficient (based on higher heating value) limited by the minimum flue gas temperature to prevent acid gas corrosion of about 300xc2x0 to 400xc2x0 F. The present invention provides efficiencies of between about 90% and about 95% (higher heating value) for a similar system. The increased efficiency is due to the fact that the retentate stream can be cooled to much lower temperatures than the exhaust temperatures from air-fired systems. This allows much more of the heat input to the retentate to be recovered as compared to conventional air-fired systems. The efficiency can be increased still further if a condensing heat exchanger is used to further cool the flue gas stream; an option that is not typically available to air-fired units.
In embodiments of the present invention in which the compressor is powered by the expander, still further advantages are realized not only in the energy savings, but also in the fact that the separation can be solely driven by the pressure produced by the compressor without a purge that would tend to dilute the oxygen. The use of a pre-heater that uses the heat contained within the expanded retentate stream raises the thermal efficiency of a method or system in accordance with the present invention.
An additional advantage of the present invention is that the indirect heat exchange within the heat consuming device allows the use of fuels that contain significant amounts of inorganics because the ceramic membrane system can thereby operate without any combustion products ever entering the membranes. In a method and system contemplated by the present invention, the operation of the heat consuming device and the ceramic membrane system are somewhat decoupled. The heat consuming device can be operated without the oxygen production system, which aids in startup and shut down of the entire system. In this regard, in the event of a failure of the ceramic membrane system, the heat consuming device can still be operated using a backup oxygen supply. Further, turndown of the ceramic membrane system can be handled independently of the operation of the heat consuming device.